This invention relates to filters and to a method for making filters which remove one or more substances, such as fine solids, from a fluid, i.e., a liquid or a gas. More particularly, this invention relates to metal filters and a method for making such filters useful under adverse conditions such as corrosive environments at elevated temperatures and pressures.
Metal filters have long been used for a variety of applications. For example, porous stainless steel filters prepared from sintered metal particulate, e.g., stainless steel powder, have found use in a variety of processes where high pressure drops are acceptable and in applications where relatively fine filtration capability must be combined with mechanical strength, resistance to high temperatures, and/or resistance to chemical attack. While presently available stainless steel filters are acceptable for many such applications, they and other presently available filters suffer from severe shortcomings in certain hostile environments, e.g., high temperature processes for treatment of sulfur-containing fossil fuels such as sour crude and some coals, catalytic cracking, filtration of contaminated hydrocarbons, stack scrubbing systems, etc.. Sulfur containing environments prove to be particularly corrosive and destructive to ordinary stainless steel filter structures.
Conventional metal filters, typically prepared from austenitic stainless steel, e.g. 300 Series, rapidly corrode at elevated temperatures in the presence of sulfur due to the relatively high surface area and permeable nature of a filter structure vis-a-vis a pipe or vessel wall. The skilled artisan familiar with corrosion chemistry is aware that sulfur invasion between the grain structure of the steel leads to reactive formation of iron sulfates, iron sulfites, pyrites, etc., which in turn speed the decomposition of the exposed steel structure. In short, such filters do not have a long on-stream service life in high temperature oxidizing environments, reducing sulfidation environments, and fluid catalyst cracking applications.
Since processes such as the treatment of fossil fuels, are typically carried out at elevated pressures as well as elevated temperatures, the potential for extremely serious damage to equipment or even catastrophic failure is always attendant in such processes. A particular application where a filter structure is subjected to a high level of stress is in processes which utilize a blow back technique for periodic cleaning of the filter surface by reverse flow or jet pulsing. Differential pressures of 0.1 to 20.0 pounds per square inch (psi) (7.0 to 1410 g/cm2), and more typically, 0.1 to 10 psi (7.0 to 705 g/cm2) are used in the forward and reverse directions for carrying out the blow back process to dislodge solids which have formed on the surface of the filter. When such a process is carried out in a system where the filter is continuously exposed to high temperatures with sulfur present, the potential for catastrophic failure is enhanced.
This invention is directed to a metal filter with excellent resistance to deterioration/corrosion in applications where high temperatures and high pressures are encountered in concert with adverse chemical environments, such as high sulfur levels.
A porous metal filter possessing enhanced anticorrosive properties is provided by the present invention which includes sintered metal particles composed of an alloy composition of iron aluminide having a range in concentration corresponding to Fe3Al to FeAl phases and having a Bubble Point ratio of about 1.6 or less.
An object of this invention is to provide a filter with the requisite properties for such applications and which is capable of withstanding high differential pressures and particularly the severe conditions encountered in the course of such filter being periodically subjected to repeated reverse flow (blow back) or jet pulsing to clean the filter and purge solids which have formed on the filter from the system.
In accordance with the subject invention, a porous metal filter is provided and particularly a seamless, porous metal filter is provided which substantially overcomes the limitations described above with regard to presently available metal filters. Filters in accordance with the subject invention have substantially uniform pore characteristics with concomitant longer on-stream life and uniform blow back characteristics, making them particularly desirable in gas filtration applications where the hostile environment encountered may result in catastrophic failure of conventional metal filters. The metal filters in accordance with the subject invention may have different forms and configurations, but are preferably seamless, hollow or cylindrical, porous structures, preferably of substantially uniform diameter, thickness, and pore structure, comprising sintered metal particulate of a corrosion resistant, iron aluminide alloy, of the type described below, in which the individual alloy particles are bonded to each other. The porous metal articles preferably have Bubble Point ratios of 1.6 or less.
While corrosion resistant alloy compositions of the invention correspond to iron aluminide compositions having Fe3Al to FeAl phases, the preferred alloy compositions include iron, aluminum, chromium, and boron. Other elements such as molybdenum, magnesium, niobium, zirconium, titanium, vanadium and yttrium and rare earth elements can be added to enhance different alloy performance characteristics. Some alloys which are suitable for use in the present invention are of a type described in McKamey et al. U.S. Pat. No. 4,961,903, incorporated herein by reference.
The preferred method of manufacture contemplates formulating a stabilized, preferably thixotropic, suspension of iron aluminide powder. The preferred cylindrical, seamless metal filter may be formed by pouring the resulting slurry into a ceramic core which is spun to compact the powder against the interior wall of the core. After further processing, the formed tube is dried, optionally densified, and vacuum sintered. The sintered tubes are then fabricated into filters in conformity with the teachings of Koehler, U.S. Pat. No. 4,822,692, incorporated herein by reference.
Other objects and advantages of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific embodiments are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent from the detailed description.